Radioactive waste disposal package

ABSTRACT

A radioactive waste disposal package comprising a canister for containing vitrified radioactive waste material and a sealed outer shell encapsulating the canister. A solid block of filler material is supported in said shell and convertible into a liquid state for flow into the space between the canister and outer shell and subsequently hardened to form a solid, impervious layer occupying such space.

BACKGROUND OF THE INVENTION

This invention relates generally to a waste container and, moreparticularly, to a container or package for storing radioactive wastematerials for long periods of time. The U. S. Government has rights inthis invention pursuant to Contract No. E512-06400 between the U. S.Department of Energy and Westinghouse Electric Corporation.

The safe and effective management of radioactive waste is one of themost critical problems facing the nuclear industry today. In view of themany hazards and problems associated with the management of high-levelradioactive waste, numerous Federal Regulations have been promulgatedand/or are being developed to govern the permanent storage of such wastein containers for burial in suitable repositories that have yet to beselected. A major task resides in the development of a container orcomprehensive waste package capable of safely containing such waste in amanner consistent with the above-mentioned regulations. In accordancewith these regulations, the waste package, in addition to satisfyingradiation, criticality, fire and handling safety requirements, mustprovide for: (1) positive containment of the radionuclides until theactivity of the material decays significantly (estimated at about 1,000years), (2) the release of radioactive material after theabove-mentioned containment period to the geologic setting at a gradualrate sufficiently retarded to permit the barriers provided by theunderground location and natural geologic materials to protect man andhis environment, and (3) the retrievability of the waste material for asignificant period of time (about 150 years) after initial emplacement.

Various attempts have been made to design waste packages meeting theabove criteria. Some of these efforts include multi-layeredconstructions formed of inner and outer shells to minimize the releaserate of radionuclides in the event that the outer shell or containmentbarrier, often referred to as an "overpack", is breached. A seriousproblem associated with these known multi-layered packaged constructionsis that the outer shell or overpack is not adequately supportedinternally to prevent outer shell deformation resulting from theexternal pressure loads associated with repository conditions. Also,these known waste packages generally are vulnerable to crushing forcessince they lack the necessary compressive strength to resist externalpressure loads.

Accordingly, it is a primary object of the present invention to overcomethe above noted shortcomings by providing a new and usefulself-stabilizing radioactive waste disposal package.

It is another object of this invention to provide a radioactive wastepackage utilizing the compressive strength of the waste material toresist the external crushing forces resulting from dynamic repositoryconditions.

It is still another object of the present invention to provide a fullyinternally supported radioactive waste package preventing deformation tothe outer shell of the package.

It is a further object of this invention to provide the foregoingradioactive waste package with a self contained filler completelyoccupying all spaces and voids between the inner and outer shells of thepackage.

These and other objects, advantages, and characterizing features of thepresent invention will become clearly apparent from the ensuing detaileddescription of an illustrative embodiment thereof, taken together withthe accompanying drawings wherein like reference characters denote likeparts throughout the various views.

SUMMARY OF THE INVENTION

A waste package comprising a sealed canister for containing radioactivewaste material in the form of an inert, insoluble glass productcontaining radionuclides. The canister is disposed within a sealed shellor container in radially spaced relation thereto and a metallic fillercompletely occupies the annular space between the canister and thecontainer. The filler material is initially cast in solid form at oneend of the canister and subsequently heated to liquefy the fillermaterial for flow into the above mentioned space.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a self-stabilizing radioactivewaste disposal package constructed in accordance with this invention;

FIG. 2 is a fragmentary sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a vertical sectional view, showing the initial stage ofassembling the waste package of this invention; and

FIG. 4 is a view similar to FIG. 2, showing the final stage of assemblyin fabricating the waste package of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in detail to the illustrative embodiment depicted in theaccompanying drawings, there is shown in FIG. 1 a self stabilizeddisposal container or package, comprehensively designated 10, adapted tocontain and store radioactive waste material including spent nuclearfuel for long periods of time. The package 10 also is constructed in amanner facilitating the manipulation thereof by suitable remote handlingapparatus, as well as meeting the criteria for the safe transport ofsuch materials.

As shown in FIG. 1, the waste package 10 comprises an inner, speciallyreinforced canister 11 completely encapsulated by a corrosion resistantmetallic shell 12, often referred to as an "overpack". The reinforcedcanister 11 is fabricated by welding together the necessary componentparts or segments that have been prefabricated from 304 L stainlesssteel into a cup-shaped, cylindrical body 13 having a cylindricalsidewall 15 and an inwardly dished or concave bottom wall 16 formedintegral with the sidewall 15. In lieu of 304 L stainless steel, thecanister body 13 can be formed of carbon steel or Ti Code -12 material,if desired, to reduce costs or extend the corrosion life of thecanister.

The upper end of canister 11 is provided with a top wall 17 in the formof a thick circular plate welded, as at 18, to the upper end of body 13and formed with a central opening 20 to provide access to the interiorof the canister 11. The canister 11 terminates at its upper end in aneck portion 21 welded or otherwise fixedly secured to top wall 17adjacent the opening 20. The neck 21 is formed with a throat portion orbore 22 tapering outwardly adjacent its lower end to merge with theopening 20 and form therewith a continuous passage or inlet 23 for theflow of waste material into the interior of canister 11. The uppersurface of top wall 17 adjacent the periphery thereof is formed with anannular recess 25 for a purpose that will hereinafter be more fullyexplained.

A significant feature of this invention resides in strengthening thecanister 11 sufficiently to react radial and axial forces generated byexternal pressure loads. To this end, the upper portion of body 13,which is devoid of waste material to form an empty space or overfillvolume 26, is reinforced while the bottom portion of body 13 takesadvantage of the waste form to provide the desired reactive compressivestrength. Accordingly, the upper portion of body 13 is formed with awall portion 15a of substantially greater thickness than the remainderof wall 15 to reinforce the same. For the same reason, the top wall 17also has a thickness approximating the thickness of the sidewall portion15a. It should be understood that the length of thickened wall portion15a is determined by and approximates the depth of the unoccupied space26 to provide the desired reinforcement for this portion of the canister11.

The bottom portion of body 13 houses the radioactive waste material inthe form of glass 27 and retains the glass in a shape that can reactexternal pressure loads even if the glass develops cracks. As is wellknown, radioactive waste materials can be converted into an inert, solidform by a glass vitrification process in which the radionuclides arecontained within the formed glass product. Since this technique is wellknown, no further description or amplification thereof is believednecessary. However, the canister 11 of the present inventionadvantageously utilizes this glass form of radioactive waste to enhancethe compressive strength of the canister 11.

In order to retain the shape of the glass, an internal stiffener, in theform of a concave disk 28, is provided at the top of the waste glass 27.The stiffener disk 28 is welded or otherwise fixedly secured along itsperipheral edge 30 to the lower portion of the thickened wall portion15a and is formed with a central opening 31 to pass the molten wasteglass therethrough during the canister filling operation. Thisstiffener, in conjunction with the canister sidewall 15 and bottom wall16, serves to structurally encapsulate or confine the entire body ofwaste glass. This encapsulated glass, even if cracked, possesses highcompressive strength and serves to react any external pressure loadsacting against the lower portion of the canister 11. Thus, thecombination of the thickened wall at the upper empty end of the canisterand the waste glass deposited in the remainder of the canister offerssufficient compressive strength to withstand any external pressure loadsresulting from dynamic repository conditions.

The top wall 17 of canister 11 supports along its periphery a collarassembly, generally designated 32, having an end wall 33 and acylindrical skirt or sidewall 35. The end wall 33 is provided with acentral opening 34 for accommodating the neck 21 of canister 11. As bestshown in FIG. 2, the lower end of the skirt 35 has a castellated ortooth-like formation comprising alternating recesses 36 and projections37. The projections 37 are received in annular recess 25 and suitablyaffixed thereto, while the recesses 36 define openings in the skirt 35.The collar assembly 32 also houses a thick, circular block of solidfiller material 38, preferably lead, cast or otherwise fixedly securedto the end wall 33 and skirt 35 and disposed about the neck 21. Thebottom of the block of filler material 38 is slightly spaced above theupper surface of canister top wall 17 to define assembly clearance andan annular passage 40 communicating with the recesses 36 for a purposethat will hereinafter become apparent.

A tapped bushing 41, located within the upper end of inlet 23, is weldedto the inner surface of neck 21 after the canister is filled with thewaste product to seal the canister closed. The bushing 41 is formed witha tapped opening 42 for receiving the threaded shank 43 of a pintle 45subsequently utilized for lifting or otherwise handling the wastepackage by suitable remotely controlled handling apparatus (not shown).

The canister 11 is completely enveloped by the outer shell 12, whichcomprises a cylindrical side wall 46, a dished, concave bottom wall 47complementary in shape to the bottom wall 16 of canister 11, and a topclosure plate 48 to which is rigidly secured the pintle 45. The topclosure plate 48 is welded, as at 50, along its peripheral edge to theshell side wall 46. An annular, titanium backing ring 51 is positionedadjacent the juncture of the closure plate periphery and cylindricalside wall 46 to facilitate welding. The thin outer shell 12 preferablyis formed of corrosion resistant titanium to provide the necessarylong-term resistance to corrosion and other deleterious effectsencountered in underground repository sites.

An important feature of the present invention resides in the provisionof a low melting point, self contained, filler material 38 disposed insolid form in the collar assembly 32. After the complete waste package10 is sealed closed by welding the closure plate 48 onto the shell 12,the upper portion of the package is heated to melt and liquefy thefiller material 38 and cause it to flow via passage 40 and the openingsdefined by recesses 36 into the narrow, annular space 29 betweencanister 11 and shell 12. In its molten state, the filler materialcompletely fills this annular space, positively eliminating any voidsbetween the canister 11 and shell 12. After heating, the filler materialcools and solidifies to provide structural support for the thin shell 12and stabilize it against buckling or deformation from external pressureloads. Additionally, the metal filler material offers thermal conductionto facilitate the flow of decay heat from canister 11 to shell 12, whichis then dissipated into the repository environment. Moreover, the leadfiller serves as a gamma and neutron shield for attenuating the neutronsand gamma rays emitted from the radioactive waste material in thecanister 11.

In assembling the waste package 10, molten radioactive waste glassmaterial is poured through inlet 23 into the fabricated canister 11 upto the level of stiffener disk 28 with any foaming of the molten glassbeing accommodated in the volume 26. After filling the canister, thecontents are allowed to cool and harden. The canister 11 is then sealedclosed by resistance welding the bushing 41 in place within neck 21. Thesealed canister 11 can now be inserted into the shell 12.

As shown in FIGS. 3 and 4, the overpacking operation preferably isaccomplished by placing an empty shell 12 in a cylindrical holdingfixture, generally designated 52, located in a hot cell. The fixture 52includes an assembly pit 53 having a cylindrical cavity 55 formedtherein. While the assembly pit 53 shown is formed of concrete below thefloor level of the hot cell, it should be appreciated that the pit canbe formed of other suitable materials including carbon steel anddisposed above the cell floor level, if desired. A cylindrical sleeve 56of suitable insulation is placed in the cavity of pit 53 and, in turn,receives a positioning sleeve 57 for holding and maintaining an emptyshell 12 (FIG. 3) in the proper upright attitude. The sealed canister11, along with the collar assembly 32, is then lowered into the shell 12with nominal clearance provided between the canister peripheral surfaceand the inner surface of shell 12 to facilitate easy entry thereinto.The projections 37 of the collar assembly skirt 35 disposed within theannular recess 25 of canister top wall 17 serve to accurately center thecollar assembly 32 relative to the canister 11. The top closure plate48, which includes the pintle 45, is then brought into position andwelded onto the shell 12.

After closure is completed, the head or upper portion 58 (FIG. 4) of thefixture 52 is lowered by means of a lifting device 60 into place. Thishead portion of the fixture 52 includes an insulation cap 61 and apositioning head 62 adapted to mate with the insulation sleeve 56 andpositioning sleeve 57, respectively. A heater coil 63 is mounted withinthe skirt of positioning head 62 and is energized for raising thetemperature of the package 10 sufficiently, approximately 400° C., tomelt the lead filler material 38. The lead liquefies and flows viapassage 40 and recesses 36 into the annular space 51 and completelyfills the same to eliminate any voids between the canister 11 and shell12. Upon completion of this heating step, the head portion 58 is liftedand the package 10, after cooling, is removed from the assembly pit andavailable for emplacement in an underground repository.

While lead preferably is employed as the filler material because of itslow melting point (327° C.) and its gamma and neutron shieldingproperties, it should be appreciated that any suitable filler materialpossessing similar properties can be used in lieu of lead, if desired.The reason for liquefying the lead after complete closure is toeliminate any problems associated with lead vapor in an open system.

From the foregoing, it is apparent that the objects of the presentinvention have been fully accomplished. As a result of this invention, anew and useful self-stabilizing radioactive waste disposal package isprovided for meeting all of the requirements of Federal Regulations asto integrity, long-term radionuclide containment, gradual radionucliderelease after the containment, and retrievability. By taking advantageof the waste product form, together with the reinforcement of certainportions of the canister, sufficient compressive strength is realized toeffectively resist the external crushing forces associated withanticipated dynamic repository conditions. Internal support for theouter shell to preclude buckling or deformation thereof is provided by aself-contained solid filler material convertible after completelysealing the package into a liquid state for filling the space betweenthe canister and shell to positively eliminate any voids therebetween.

The foregoing description of a preferred embodiment of this inventionhas been presented for purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed, and obviously many modifications and variations arepossible in light of the above teaching. The embodiment was chosen anddescribed in order to best explain the principles of this invention andits practical application to thereby enable others skilled in the art toutilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

I claim:
 1. A waste package comprising: a canister for containingvitrified radioactive waste material, said canister having a side wall,an end wall terminating in a neck for defining an inlet, a closuremember sealing said inlet closed, a sealed outer shell completelyencapsulating said canister in radially spaced relation to said canisterside wall and defining a space therebetween, means adjacent said neckfor supporting a metallic filler in solid form, said metallic fillerconvertible into a liquid state for flow into said space after saidouter shell has been sealed and then subsequent hardened to form a solidlayer completely occupying said annular space.
 2. A waste packageaccording to claim 1, wherein said canister comprises a side wall andopposite end walls, said vitrified radioactive waste material occupyingthe major portion of said canister from one of said end walls to apredetermined level spaced from said other end wall and offeringresistance to external pressure loads.
 3. A waste package according toclaim 2, wherein said side wall is formed with a reinforced portionextending from said level of vitrified radioactive waste material tosaid other end wall.
 4. A waste package according to claim 1, includingmeans for retaining said vitrified radioactive waste material in adesired shape.
 5. A waste package according to claim 4, wherein saidretaining means comprises a disk secured about the periphery thereof tosaid sidewall for confining in conjuction with said sidewall and saidone end wall said vitrified radioactive waste material in said desiredshape.
 6. A waste package according to claim 1, including a collarassembly mounted within said shell about said canister neck.
 7. A wastepackage according to claim 6, said collar assembly supporting said solidfiller material prior to converting said filler material into a liquidstate.
 8. A waste package according to claim 7, wherein said collarassembly comprises a skirt having a castellated edge supported on anexternal surface of said one end of said canister.
 9. A waste packageaccording to claim 8, wherein said castellated edge defines openingsestablishing communication with said space for flow of said fillermaterial when in a liquid state.
 10. A waste package according to claim1, wherein said shell is formed of a corrosion resistant material.
 11. Awaste package according to claim 10, wherein said corrosion resistantmaterial is titanium.
 12. A waste package according to claim 1, whereinsaid metallic filler is lead.